Toxin domain:PIN. This PIN (PilT N terminus) domain is a compact domain of about 100 amino acids. The domain has two nearly invariant aspartates. The function of the PIN domain is unknown but a role in signaling appears likely given the presence of this domain is StbB and DIS3. The majority of PIN-domain proteins found in prokaryotes are the toxic components of toxin-antitoxin operon, and toxin-antitoxin gene cassettes, though first found in plasmids, are found to be abundant in free-living prokaryotes, including many pathogenic bacteria. These loci provide a control mechanism that helps free-living prokaryotes cope with nutritional stress. In many genomes PIN domains are annotated as VapC and are part of the VapBC toxin-antitoxin operons. These operons are expanded in number in several unrelated micro-organisisms including Mycobacterium tuberculosis and Sulfolobus solfataricus.